Absorbance to Wavelength Calculator

Example mapping (illustrative). Real values depend on dye, detector, and calibration.

Absorbance is defined by the Beer–Lambert relationship: A = ε · l · c. It depends on molar absorptivity (ε), path length (l), and concentration (c).

Wavelength cannot be uniquely determined from absorbance alone. To estimate wavelength, you must supply a model that maps absorbance to wavelength, such as:

• Linear: λ = m·A + b
• Two‑point: derive m = (λ2−λ1)/(A2−A1), then b = λ1 − m·A1
• Polynomial: λ = c0 + c1·A + c2·A² + c3·A³
• Table: linear interpolation between adjacent calibration pairs

Derived outputs include transmittance T% = 100·10^(−A), frequency f = c/λ, and photon energy (approx.) E(eV) ≈ 1239.84/λ(nm).

Selecting a useful wavelength range

UV–Vis spectrophotometers commonly scan about 190–1100 nm. Ultraviolet (190–400 nm) highlights many aromatic and peptide bonds, while visible light (380–740 nm) supports colorimetry and dye work. Near‑infrared bands above 740 nm are useful for some coatings and plastics. Most routine scans use 1–2 nm bandwidth and 1 nm step size, but microplate readers may use wider filters, which changes the A‑to‑λ mapping. Record the units in nm and keep temperature steady during scans.

Why absorbance alone is not enough

A single absorbance value does not uniquely identify wavelength. Different molecules can produce the same A at different λ, and even the same molecule can show multiple peaks. This calculator therefore treats wavelength as an estimated output from your chosen calibration model.

Build a calibration curve from standards

Measure absorbance for standards at known wavelengths, then fit a relationship between A and λ. A linear fit is often acceptable across a narrow region near λmax, while a polynomial can capture curvature across a wider band. Two‑point calibration is a fast check when only two trusted points exist.

Using a table for interpolation

If you have a lookup table, paste absorbance–wavelength pairs and the tool performs linear interpolation between adjacent points. For smoother behavior, include at least five pairs and keep them ordered. Add points around steep regions where A changes quickly with wavelength.

Good measurement targets and limits

Aim for absorbance between 0.10 and 1.50 for most benchtop instruments. Above about 2.00, stray light and detector limits can flatten peaks and distort calibration. Use a proper blank, keep the cuvette clean, and maintain a consistent path length, commonly 1.00 cm.

Interpreting the extra outputs

Once wavelength is estimated, the calculator reports derived quantities. Transmittance follows T% = 100·10^(−A), so small changes in A can mean large changes in T%. Frequency is computed from f = c/λ, and photon energy uses E(eV) ≈ 1239.84/λ(nm), convenient for quick comparisons.

Quality checks before exporting

Verify that your model matches the same instrument settings used to collect the calibration data, including slit width, spectral bandwidth, and integration time. If you enable extrapolation, treat results as provisional and confirm with a scan. Export CSV for lab notebooks and PDF for sharing.

Do I need calibration to estimate wavelength?

Yes. Absorbance alone cannot determine wavelength uniquely. Provide a fitted line, two-point reference, polynomial coefficients, or an absorbance–wavelength table from your own standards so the calculator can map A to λ.

Which method should I pick first?

Start with table interpolation if you have measured pairs across the range. Use linear or two‑point when the relationship is approximately straight over a narrow band. Choose polynomial only when a curved fit is justified by your data.

Why is the wavelength negative or extremely large?

That usually indicates incorrect coefficients, swapped units, or a model applied outside its valid range. Reduce extrapolation, add more calibration points, and confirm that your wavelength values are in nanometers.

What does Transmittance (%) represent here?

It is computed from absorbance using T% = 100·10^(−A). For example, A = 1.00 corresponds to about 10% transmittance. This helps relate optical density to the fraction of light passing through the sample.

How accurate is the estimate?

Accuracy depends on your calibration quality, instrument stability, and sample behavior. Use standards measured on the same instrument, keep absorbance in a reliable range, and verify results by scanning near the predicted wavelength.

Is extrapolation recommended?

Use extrapolation only as a temporary estimate. Real spectra can curve or shift, so values outside the table range may be misleading. If you must extrapolate, validate with an additional standard or a quick wavelength scan.